Corrosion resistant honeycomb

ABSTRACT

Corrosion resistant metallic honeycomb composed of a plurality of honeycomb cells having cell walls that include cell edges that form the edge of the honeycomb. A corrosion resistant coating that contains polyamideimide is used to cover the cell walls and cell edges. The corrosion resistant coating is preferably applied after the honeycomb structure has been formed.

This application is a divisional of U.S. patent application Ser. No.11/476,964, which was filed on Jun. 27, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to metallic honeycombs and theways in which such honeycombs are protected against corrosion. Moreparticularly, the present invention is directed to improving thecorrosion resistance of metallic honeycomb that is intended for use incorrosive environments.

2. Description of Related Art

Honeycomb structures are well known and widely used in many applicationswhere a high strength and low weight material is required. The combinedfeatures of low weight and strength found in honeycomb structures makesthem particularly well suited for use in aircraft and other applicationswhere high strength and low weight are particularly desirable. Honeycombstructures have been made from a wide variety of materials includingmetals, such as aluminum and aluminum alloys. Such structures arereferred to as aluminum honeycomb or aluminum core. When the term“aluminum” is used by itself herein, it is understood to cover bothaluminum and aluminum alloys. Composite materials made fromresin-impregnated fibers and papers have also been widely used inhoneycomb structures.

One common process for fabricating honeycomb structures involves bondingmultiple thin sheets of aluminum together along specially oriented nodelines. The node lines are offset between the different layers in such away that a honeycomb structure is formed when the layers are expanded.This type of process is commonly referred to as the “expansion” process.The expansion process is not suitable for fabricating honeycombstructures in some instances where certain types of aluminum are used.For example, sheets that are relatively thick or are made from certainalloys of aluminum are too stiff and cannot be formed into honeycombstructures using the expansion process.

A fabrication process or method commonly referred to as the“corrugation” process has been used to form high strength honeycombstructures in those situations where the expansion process is notsuitable. The corrugation process involves initially shaping sheets ofaluminum into a corrugated configuration. The corrugated aluminum sheetsare then bonded together along node lines to form the final honeycomb.

In both the expansion and corrugation processes, adhesives are typicallyused to bond the aluminum sheets together. Such adhesives are commonlyreferred to as “node adhesives”. The surfaces of the aluminum sheets areusually coated with a polymer-based coating or otherwise treated toinsure good adhesion between the node adhesive and the sheet. Thestrength of the bond between the adhesive and the treated aluminumsheets is generally determined by measuring the peel strength of theadhesive. Peel strength is typically measured using a standard procedureas set forth in ASTMD 1781. The test basically involves gluing two3-inch (7.6 cm) or two 1-inch (2.54 cm) wide strips of metal sheetstogether and the measuring of the amount of force required to peel thetwo strips apart.

Achieving maximum peel strength for a given adhesive is an importantgoal when manufacturing high strength honeycomb. Failure of the adhesivebond between aluminum sheets can result in complete failure of theentire structure. Accordingly, there is a need to provide surfacecoatings and/or treatments for aluminum that enhance adhesion of thenode adhesive to the metal surface.

Aluminum honeycomb is a popular core material for use in theconstruction of sandwich panels where the honeycomb is “sandwiched”between two sheets of material that are commonly referred to as “skins”.An adhesive is typically used to bond the edges of the honeycomb to theskins. In some cases, the skins are self-adhesive and do not require theuse of an adhesive layer. The strength of the adhesive bond between theedge of the honeycomb and the skins is also an important considerationin treating or otherwise coating the aluminum core. Failure of the edgeadhesive also can result in failure of the structure.

Aluminum is also usually treated to increase resistance to corrosion.Chromate has been a popular corrosion resistant coating for aluminumthat has been widely used. However, there are a number of environmentalconcerns associated with the use of chromate. A number of othercorrosion resistant coatings have been developed for aluminum. Forexample, U.S. Pat. No. 3,687,882 describes coating aluminum withsilane-titanante dispersions. Anodizing the aluminum with phosphoricacid and/or coating the aluminum with epoxy or modified phenolic havealso been popular methods for making the honeycomb corrosion resistant.

In the art of preparing aluminum honeycomb, there has been and continuesto be a need for surface treatments that not only provide good peelstrengths at the nodes and edges, but also provide good corrosionprotection, especially when the honeycomb is intended for use in knowncorrosive environments. Developing a coating system or treatment thatprovides all of the above features is an important goal in makinghoneycomb from aluminum that is suitable for use where high strength andlight weight is required.

SUMMARY OF THE INVENTION

In accordance with the present invention, corrosion resistant metallichoneycomb is provided wherein the honeycomb edges and/or walls arecoated with a corrosion resistant coating that contains polyamideimide.In addition, the present invention covers methods for applying thecorrosion resistant coating to the surfaces of the metallic materialboth before and/or after formation of the metallic material into thehoneycomb.

In accordance with the present invention, it was discovered that thecorrosion resistance of metallic honeycomb is increased by covering theedges and/or walls of the honeycomb with a corrosion resistant coatingthat contains polyamideimide. It was further discovered that thecorrosion resistant coating is preferably applied to the honeycomb afterthe honeycomb has been formed in order to fill micro-cracks that areformed during processing of the sheets of metallic material into thefinal honeycomb structure.

The present invention involves providing corrosion resistance to ametallic honeycomb that has a plurality of honeycomb cells that includecell walls that have edges. The edges of the cell walls form the edgesof the honeycomb. As a feature of the present invention, a corrosionresistant coating that contains polyamideimide is used to cover at leastthe cell walls and preferably both the cells walls and edges. Thecorrosion resistant coating is useful in treating metallic materials,such as aluminum and aluminum alloys, which have been previouslyanodized and/or coated with a polymer to increase corrosion resistance.The corrosion resistant coating of the present invention is especiallyeffective in sealing micro-cracks that may develop in such previouslyapplied corrosion resistant coatings during formation of the honeycomb.

In accordance with the present invention, the corrosion resistantcoating can be applied to the honeycomb as a final corrosion proofingcoating during sandwich panel construction without adversely affectingthe bond between the edges of the honeycomb and the face sheets.Accordingly, the coating provides the dual benefits of increasedcorrosion resistance and good skin-core bonding characteristics.

The above described and many other features and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary corrosion resistant metallic honeycomb inaccordance with the present invention prior to application of facesheets or skins to the edges of the honeycomb to form a sandwich panel.

FIG. 2 shows the exemplary corrosion resistant metallic honeycomb afterit has been bonded to face sheets to form a sandwich panel.

FIG. 3 is a partial sectional view of the exemplary honeycomb shown inFIG. 1, which shows an edge of the honeycomb in position for bonding toone of the face sheets.

DETAILED DESCRIPTION OF THE INVENTION

The present invention involves increasing the corrosion resistance ofmetallic honeycomb by coating the exposed surfaces of the honeycomb withpolyamideimide. The invention may be used to treat honeycomb that ismade from any of the metallic materials that are used to make honeycomb,such as aluminum and aluminum alloys. Exemplary aluminum alloys include5052 and 5056 alloys, which are commonly used in honeycomb structures.Preferred honeycombs are those made from aluminum or aluminum alloys.The metallic materials used to make honeycomb are typically in the formof sheets or foil that ranges in thickness from 0.0005 inch up to 0.05inch, but may be thicker or thinner, if desired. The honeycomb can be ofvirtually any size or shape and can be made by any of the knownhoneycomb manufacturing methods, including the expansion and corrugationmethods.

The honeycomb may be used for any number of purposes. However, thehoneycomb is preferably used for making sandwich panels that find a widevariety of applications in the aerospace industry. The sandwich panelsare used as both structural and non-structural components of aerospacevehicles. The honeycomb is especially well suited for making sandwichpanels that are used in aircraft. Exemplary sandwich panels orstructures are used in engine nacelles, wing and tail structures andfuselage components, such as floorboards and other interior partitions.The corrosion resistance provided by the present invention isparticularly desirable in such aircraft components because they aretypically subjected to a wide variety of environments that arepotentially corrosive.

The three basic components of an exemplary honeycomb sandwich panel foruse in aerospace applications are shown in FIG. 1 prior to formation ofthe panel. The components include a honeycomb core 12 that has walls 11which form a plurality of honeycomb cells 13. The walls have edges thatform the faces or edges of the honeycomb as shown at 14 and 16. Theother two components are the face sheets or skins 17 and 19. The facesheets 17 and 19 include interior surfaces 21 and 23, respectively, forbonding to the honeycomb edges. The face sheets 17 and 19 also includeexterior surfaces 25 and 27, respectively. The face sheets 17 and 19 canbe made from a wide variety of metallic and/or composite materials thatare bonded to the edges of the honeycomb 14 and 16.

The dimensions of the honeycomb can be varied widely. For aerospace use,the honeycomb cores will typically have ⅛ to ½ inch (3.2 to 12.7 mm)cells (i.e. in diameter cross-section) with the cores being ¼ inch (6.4mm) to 2 inches (50.8 mm) thick (distance between the honeycomb edges).The thickness of the honeycomb walls may also be varied with typicalhoneycomb walls being on the order of 0.001 inch (0.25 mm) to 0.005 inch(0.13 mm) thick. The combination of cell size, wall thickness anddensity of the material that is used determines the weight of the core,which is expressed in pounds per cubic foot (pcf). Aluminum and aluminumalloy honeycomb having weights on the order of 2 pcf to 8 pcf arepreferred.

The face sheets 17 and 19 that are bonded to the honeycomb 12 can bemade from any of the materials that are used in the construction ofsandwich panels having metallic honeycomb cores. Such materials includeboth metals and composite material. When metals or pre-cured compositeface sheets are bonded to the honeycomb, an adhesive is used.Alternatively, uncured sheets of composite materials known as a“prepreg” may be located on the edges of the honeycomb and bonded/curedin place. Such prepregs may be self-adhesive or an additional adhesivemay be used. In FIG. 1, the face sheets 17 and 19 are preferablyself-adhesive prepreg that is cured in place to form the final sandwichpanel 10 (as shown in FIG. 2), which includes cured face sheets 18 and20.

The prepreg face sheets usually include one or more layers of fibers anda resin matrix. The fibers that are used in the prepreg face sheets 17and 19 can be any of the fiber materials that are used to form compositelaminates. Exemplary fiber materials include glass, aramid, carbon,ceramic and hybrids thereof. The fibers may be woven, unidirectional orin the form of random fiber mat. Woven carbon fibers are preferred, suchas plain, harness satin, twill and basket weave styles that have arealweights from 80-600 gsm, but more preferably from 190-300 gsm. Thecarbon fibers can have from 3,000-40,000 filaments per tow, but morepreferably 3,000-12,000 filaments per tow. Similar styles of glassfabric may also be used with the most common being 7781 at 303 gsm and120 at 107 gsm. When unidirectional constructions are used, typicalply-weights are 150 gsm for carbon and 250 gsm for glass. All of thesecarbon and glass fibers and fabric are commercially available.

The resin used in the prepreg face sheets may be any of thethermosetting polymers that are typically used for prepreg face sheets.Typical polymer resins include epoxies, cyanate esters andbismaleimides. The thermosetting resins may include thermoplasticpolymers to provide viscosity control and enhance fillet formation andtoughness, as is known in the art. Exemplary self adhesive face sheetsare described in detail in U.S. Pat. Nos. 6,440,257 and 6,508,910, thecontents of which is specifically incorporated by reference.

The corrosion resistant coating in accordance with the present inventionmay be applied to the surfaces of the metallic material either before orafter it has been formed into the final honeycomb. However, it ispreferred that the coating be applied to the metallic material after ithas been formed into the final honeycomb. It was discovered that thecorrosion resistant coating was effective in covering micro-cracks,which may develop in the surface of the metallic material and/orpre-applied corrosion coatings during the honeycomb formation process.

The coating may be applied by any of the known coating applicationsystems. The preferred application technique is to dip the honeycombinto a solution of polyamideimide to completely coat all exposedsurfaces of the honeycomb. The honeycomb may be completely immersed inthe polyamideimide solution to provide the coating or it may bepartially immersed multiple times in different orientations so as toprovide complete surface coverage. As shown in FIG. 3, both thehoneycomb walls 11 and honeycomb edges 16 are preferably covered with acoating of polyamideimide 15 in accordance with the present invention.Other exemplary application systems include spraying and overdipping.

The metallic material is preferably, but not necessarily, subjected toone or more conventional corrosion prevention treatments prior toformation of the honeycomb. Any of the known corrosion resistant surfacepre-treatments and/or coatings (primers) may be used prior toapplication of the corrosion resistant coating in accordance with thepresent invention. It is only necessary that the surface treatmentand/or primer be compatible with polyamideimide. Aluminum and aluminumalloy materials are preferably anodized with phosphoric acid and thencoated with a corrosion resistant primer. Exemplary primers includethose made from epoxy or modified phenolic resins. Exemplary primers areavailable commercially available from Cytec Corporation (Anaheim,Calif.) under the tradename BR 127. Conversion type coating such asAlodine® may also be used.

Aluminum honeycomb that is suitable for treatment in accordance with thepresent invention is commercially available from Hexcel Corporation(Dublin, Calif.) under the tradenames HexWeb®CR-PAA, HexWeb®CRIII andHexWeb®ACG®. HexWeb®CR-PAA is an aluminum honeycomb that has beenanodized with phosphoric acid and coated with a modified phenolicprimer. HexWeb®CRIII is an aluminum honeycomb that has been conversioncoated with chromate and then seal coated with titanate and aminosilane. HexWeb®ACG® is an aluminum honeycomb that has been conversioncoated with chromate and then seal coated with titanate and aminosilane.

The corrosion resistant coating 15 in accordance with the presentinvention is composed substantially of polyamideimide. Minor amount ofother ingredients are possible. However, it is preferred that at least95 weight percent of the coating be polyamideimide. Even morepreferably, the coating should be at least 99 weight percentpolyamideimide. The coating 15 should be from 0.0002 inch to 0.0004 inchthick on the cell wall. It is preferred that the coating 15 be as thinas possible to keep the weight of the honeycomb as low as possible,while at the same time providing complete coverage of the exposedsurfaces of the honeycomb. The exposed surfaces are those that are notlocated in the nodes where the various metallic layers are bondedtogether to form the honeycomb.

The polyamideimide coating 15 is formed by applying a solution ofreactive (amic acid) prepolymer to the honeycomb and then heating thehoneycomb to achieve drying (i.e. removal of the solvent) and curing.The reactive amic acid polymer solution will typically contain aromaticamide, aromatic amic acid and aromatic imide moieties. These moietiesare readily soluble in NMP. A preferred exemplary reactive amic acidpolymer is TORLON® AI-10, which is available from Solvay AdvancedPolymers (Alpharetta, Ga.). TORLON® AI-10 is supplied as a yellow powderin the un-imidized or amic acid form. TORLON® AI-10 has an acid number(mg KOH/g of polymer) of 80 and is soluble in diprotic solvents. Othersuitable (amide-imide) prepolymers solutions are set forth in U.S. Pat.Nos. 4,316,974 and 5,087,658.

The (amide-imide) prepolymer is soluble in a wide variety of organicsolvents including: 1-methyl 2-pyrrolidone (NMP); formamide; dimethylsulfoxide; dimethyl acetamide The amic acid polymer is dissolved in thesolvent to provide a solution that has the appropriate viscosity forapplying the desired coating thickness depending upon the applicationtechnique. When using the preferred technique of dipping the honeycombin the solution, the solution will typically contain from 5 to 30 weightpercent amic acid polymer in the solvent. Preferred dipping solutions,in which the honeycomb is immersed, contain from 5 to 17 weight percentTORLON® AI-10 powder in NMP solvent.

After the honeycomb is dipped in the solution of amic acid polymer, itcan be dried at temperatures up to about 200° C. in order to removesolvent and form an uncured coating that is subsequently heated to atemperature of between 200 to 225° C. in order to provide polymerization(curing). Alternatively, the coating of polymer solution may beimmediately heated to the curing temperature to provide both drying andcuring of the polymer solution. In either case, the honeycomb with thecured polyamideimide coating is then ready for bonding to the facesheets 17 and 19 without further treatment other than the application ofan adhesive to the edges of the honeycomb, if required, for bonding tothe face sheets.

Examples of practice are as follows:

EXAMPLE 1

Two aluminum alloy honeycombs (4 inches wide, 4 inches long and 0.63inch thick) were coated with a corrosion resistant coating in accordancewith the present invention as follows: the honeycombs were HexWeb®CR-PAA™ honeycombs that are available from Hexcel Corporation (Dublin,Calif.). The honeycombs were made from 5052 or 5056 aluminum alloy. Thenominal cell sizes were 3/16 inch, the foil thickness was 0.0015 inchand the nominal density was 4.4 pounds per cubic foot (pcf). Aspreviously mentioned, the aluminum foil used in HexWeb® CR-PAA™honeycomb is phosphoric acid etched and coated with a modified phenolicprimer prior to honeycomb formation in accordance with the manufacturersestablished production process.

The HexWeb® CR-PAA™ honeycombs were immersed in a solution thatcontained 8.5 weight percent of TORLON® AI-10 polymer dissolved in 91.5weight percent of NMP solvent. The honeycombs were removed from thesolution and the excess solution allowed to drip from the honeycomb forto form a “wet” honeycomb. The “wet” honeycomb was then heated to atemperature of about 200° C. for about 10 minutes to dry and cure thepolymer. The resulting coatings of polyamideimide ranged from about0.0002 to 0.0004 inch in thickness.

The two honeycombs with the polyamideimide coating were then subjectedto thermal aging according to a 10-day acidified salt spray testingaccording to ASTM G85-98 The polyamideimide coated cores had an averageof 2 holes (perimeters of about 1 mm) and 22 average size (perimeters of1 mm to a few mm) corrosion spots. Observations were based on visualinspection of two specimens. The entire core was observed for the holecount while only the perimeter of the core was inspected for corrosionspots.

COMPARATIVE EXAMPLE 1

Two HexWeb® CR-PAA™ honeycomb were subjected to the same acidified saltspray testing as the honeycombs of Example 1. The honeycomb was the sameas in Example 1, except that the honeycomb was not coated withpolyamideimide in accordance with the present invention. After acidifiedsalt spray testing, the two cores were inspected in the same manner asExample 1 and found to have an average of 4 holes (perimeters of about 1mm) per core and 80 average size corrosion spots and some additionalmuch smaller corrosion spots (perimeters of less than 1 mm).

COMPARATIVE EXAMPLE 2

Two honeycombs made from 5052 or 5056 aluminum alloy and available fromHexcel Corporation under the tradename HexWeb®CRIII were subjected tothe same acidified salt spray testing as Example 1. The honeycombs werethe same size and had the same cell size and foil thickness as thehoneycombs in Examples 1 and 2. The honeycomb was treated with achromate conversion coating and included an epoxy primer coating. Afteracidified salt spray testing, the two cores were inspected in the samemanner as Example 1 and found to have no holes per core and 47 averagesize corrosion spots.

COMPARATIVE EXAMPLE 3

The same honeycomb as used in Comparative Example 1 was overdipped withadditional modified phenolic primer after formation of the twohoneycombs. The overdipping of the two cores with modified phenolicprimer was done in the same manner as the polyamideimide coating inExample 1 to provide an additional phenolic primer coating. Afteracidified salt spray testing, the two cores were inspected in the samemanner as Example 1 and found to have one hole per core (perimeters ofabout 1 mm) and 64 small corrosion spots (perimeters of about 1 mm orless).

EXAMPLE 2

Two honeycomb cores that were the same as in Example 1 were subjected tothermal aging according to a 10-day non-acidified salt spray testingaccording to ASTM B117-90. After salt spray testing, the two cores wereinspected in the same manner as Example 1 and found to have 5 holes percore (perimeters of about 1 mm) and 47 average size corrosion spots.

COMPARATIVE EXAMPLE 4

Two honeycomb cores that were the same as in Comparative Example 1 weresubjected to thermal aging according to the same 10-day non-acidifiedsalt spray testing as Example 2. After salt spray testing, the two coreswere inspected in the same manner as Example 1 and found to have 24holes per core (perimeters of about 1 mm) and 4 average size corrosionspots. In addition, numerous areas were observed where there the primerhad been removed. These areas were silver in appearance.

COMPARATIVE EXAMPLE 5

Two honeycomb cores that were the same as in Comparative Example 2 weresubjected to thermal aging according to the same 10-day non-acidifiedsalt spray testing as Example 2. After salt spray testing, the two coreswere inspected in the same manner as Example 1 and found to have 17holes per core (perimeters of about 1 mm) and 6 average size corrosionspots.

COMPARATIVE EXAMPLE 6

Two honeycomb cores that were the same as in Comparative Example 1 weresubjected to thermal aging according to the same 10-day non-acidifiedsalt spray testing as Example 2. After salt spray testing, the two coreswere inspected in the same manner as Example 1 and found to have 30holes per core (perimeters of about 1 mm) and 12 average size corrosionspots. In addition, numerous areas were observed where there the primerhad been removed. These areas were silver in appearance. There were alsoa number of blotchy areas.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited by the above-describedembodiments, but is only limited by the following claims.

1. In a method for making honeycomb wherein sheets of metallic materialare bonded together at selected locations and then expanded to form saidhoneycomb, the improvement comprising applying a solution of reactiveamic acid prepolymer to said honeycomb to form a coating thereon andcuring said coating to provide corrosion resistant layer comprisingpolyamideimide.
 2. An improved method for making honeycomb according toclaim 1 wherein said honeycomb comprises a plurality of honeycomb cellshaving cell walls and cell edges, said method including the step ofapplying said reactive amic acid prepolymer to both said cell walls andsaid cell edges to form said coating on said cell walls and cell edges.3. An improved method for making honeycomb according to claim 1 whereinsaid sheets of metallic material are anodized or coated with a polymercoating prior to formation of said honeycomb.
 4. An improved method formaking honeycomb according to claim 3 wherein said sheets of metallicmaterial are anodized and coated with a polymer coating prior toformation of said honeycomb
 5. An improved method for making honeycombaccording to claim 1 wherein said sheets of metallic material comprisealuminum or aluminum alloy.
 6. An improved method for making a honeycombaccording to claim 5 wherein said metallic honeycomb is phosphoric acidetched aluminum or aluminum alloy.
 7. An improved method for makinghoneycomb according to claim 1 wherein said corrosion resistant layerconsists essentially of polyamideimide.
 8. An improved method for makinghoneycomb according to claim 2 wherein said corrosion resistant layerconsists essentially of polyamideimide.
 9. An improved method for makinghoneycomb according to claim 3 wherein said corrosion resistant layerconsists essentially of polyamideimide.
 10. An improved method formaking honeycomb according to claim 4 wherein said corrosion resistantlayer consists essentially of polyamideimide.
 11. An improved method formaking honeycomb according to claim 5 wherein said corrosion resistantlayer consists essentially of polyamideimide.
 12. An improved method formaking honeycomb according to claim 6 wherein said corrosion resistantlayer consists essentially of polyamideimide.